In recent years, shielding of radiation generated from wastes contaminated with radioactive substances has become an urgent issue. In various facilities such as advanced medical facilities and materials research facilities that use high-energy radiation emitted from radioactive elements, accelerators, or other sources, equipment and facilities themselves need to be protected to shield radiation. As ingredients of radiation shielding materials to shield radiation, materials with high density, including lead, iron, and concrete are considered to be effective. However, these materials have the following problems.
Although lead provides high radiation shielding effects and chemical stability, there is the problem that lead is toxic. Another problem is that lead is low in hardness which makes it soft even at a room temperature. When using lead to the location that requires strength, some sort of reinforcement member is necessary. Yet another problem is that lead has a low melting point at 327.4° C. Lead cannot retain the form as a structure under an environment of high temperature because of the low melting point.
Iron also has a problem in the limited use environment because of its low resistance to corrosion. Iron is hard to use in a salty and moist environment such as a coastal area. In addition, iron cannot be prevented from degrading over time because of rusting even under a normal environment. This means that iron is not suitable for the use as a radiation shielding material to shield isolation facilities of radioactive wastes. The reason is that maintenance of this type of isolation facilities cannot be conducted frequently because of the limited access to the facilities while such facilities are assumed to be used for a long period of time, from a few decades to a hundred years.
Concrete also has a problem in its lower density. The density is lower than, for example, the aforementioned lead and iron. In order to obtain sufficient radiation shielding effects, structures made of concrete such as a wall must be very thick. Although there is a type of concrete called “heavy concrete”, the density of which is increased by adding high-density aggregates, radiation shielding effects are still insufficient. Another problem of heavy concrete is that it contains pieces of metal and oxides as aggregates, which significantly reduces the mechanical strength and chemical stability of heavy concrete. Therefore, heavy concrete is also not suitable for the use as a radiation shielding material to shield such isolation facilities as mentioned above.
In the meantime, some technologies have been proposed for improving radiation shielding effects of concrete, mortar, or the like by increasing density through adding ferrite (for example, refer to Patent Literatures 1 and 2). Ferrite is a type of magnetic material that contains iron oxides and is widely used in various kinds of electronic components including motor magnets, toner drums for photocopying machines and laser printers, magnetic disks, and magnetic tapes. The radiation shielding materials in Patent Literatures 1 and 2 focus on the high density (radiation shielding effects) rather than the magnetic properties. In order to build a radiation shielding structure using the technologies of Patent Literatures 1 and 2, it is necessary to go through the following operations of (1) setting up a form, (2) arranging rebars in the form, (3) pouring concrete or mortar in the form, (4) curing the concrete or mortar, and (5) removing the form. The problem is that the series of such operations entail labor, time and costs.
Considering such circumstances, the applicant of the present invention has developed a molded article for structure construction with enhanced radiation shielding effects in which the molding material containing ferrite powder in a ratio of 60% by mass or more is molded into a predetermined shape and fired, whereby the fired molded article has a density of 3.5 g/cm3 or higher. A related patent application has already been filed as Japanese Patent Application No. 2013-057194. Note that a molded article for structure construction means the article such as a brick and a tile that is stacked, assembled, or bonded to one another to form a shielding structure in a construction, for example, a wall, a ceiling, and a floor. The ferrite powder used for this molded article for structure construction is preferably the one expressed by the compositional formula: AO.nX2O3, where A is at least one type of element selected from among magnesium (Mg), calcium (Ca), manganese (Mn), cobalt (Co), nickel (Ni), copper (Cu), strontium (Sr), barium (Ba), and lead (Pb), and X is at least one type of element selected from among iron (Fe), cobalt (Co), and nickel (Ni) while n is a mole ratio defined as a real number from 1 to 9.
This molded article for structure construction not only exhibits excellent radiation shielding effects but also demonstrates high strength and excellent workability. However, this molded article for structure construction has a problem in that it is not suitable for use in a facility in which equipment susceptible to magnetic fields, such as precision analytical equipment and measuring equipment, is installed. The reason is that the molded article for structure construction that contains ferrite expressed in the aforementioned compositional formula, which typically indicates hard ferrite, could adversely affect the aforementioned equipment, because it becomes permanent magnet once exposed to an external magnetic field and continues to generate the magnetic field even after the external magnetic field is eliminated.
Patent Literature 3 is another example that describes a construction structure with heavy media additives including hematite, lead, and iron, added to a layer made of concrete as a part of a structure of construction such as a wall and a ceiling (refer to Claim in Patent Literature 3). Patent Literature 3 also describes that addition of such heavy media additives increases radiation shielding effects of a construction structure (refer to Paragraph 0020 in Patent Literature 3). In the construction structure of Patent Literature 3, however, the layer to add such heavy media additives as hematite is mainly composed of concrete that is mixed in the construction site. Therefore, the construction structure of Patent Literature 3 does not go much beyond the aforementioned heavy concrete in radiation shielding effects, which is not sufficient. Another problem is that the construction entails much labor, time and costs such as setting frames.